I installed VirtualBox and Ubuntu but now I'm trying to install a software (Xilinx) but i think my machine doesn't have enough memory (the software has more than 6 Gb and it only installs until 2.5 Gb)...
How should I proceed to change the memory space? Also do you have any idea how much memory I need?
We can increase the virtual hard disk space as shown e.g. in answers to the following question:
In your case, and if it was a new installation of Ubuntu it will be far less time consuming to just start from scratch and install Ubuntu again, this time on a larger virtual drive.
If you choose a dynamically growing virtual disk you can assign it a rather huge space it will report to the guest OS. Its physical size on the host hard drive will be much smaller and it will only grow to the extent needed and never more than up to the predefined limit.